Systemic Lupus Erythematosus (SLE) is a chronic systemic disease with various clinical manifestations. It is characterized by episodic and unpredictable exacerbations and by remissions, usually associated with treatment. The challenge to rheumatologists is to reliably measure new and/or increasing disease activity, so that they can prevent permanent organ damage and premature death with timely treatment. Several reliable and valid clinical activity indices including Lupus Activity Index (LAI), Systemic Lupus Erythematosus Activity Index (SLEDAI) and Systemic Lupus Activity Measure (SLAM) exist. However,no single laboratory parameter to date has proven sufficiently reliable and valid for disease activity in SLE. Our goal is to determine the predictive value for activity/flare (using valid and reliable clinical disease activity indices:LAI, SLEDAI and SLAM) of currently available serologic tests(i.e. sedimentation rate, double-stranded DNA and complement levels) and investigational assays such as complement split products, in a longitudinal regression model that adjusts for demographic, clinical and treatment variables. The Hopkins Lupus cohort offers a unique, prospective database of demographic, social, clinical and laboratory measures that can be used to determine laboratory tests which can reliably predict disease activity and flare especially in different organ systems. This cohort began in 1987 and consists presently of 411 living patients. It is an ongoing prospective study in which SLE patients are followed by protocol with visits every 3 months. The cohort is racially balanced with over one half of the members being Afro-American and reflects a broad socioeconomic range.